1848 



HANDBOOK OF PHYSIOLOGY 



NEl'ROPIIYSIOI 1 >OY III 



SO that the cerebral respiratory quotient is 0.97 or 

 approximately unity (tables 1 and 2). In neither man 

 nor animals have an) significant differences from 

 this value been observed, even during severe meta- 

 bolic disorders such as hypoglycemia (103), diabetic 

 acidosis (99), or ketosis arising from starvation or 

 fat feeding (131 >. 



Comparison of the oxygen and glucose arterio- 

 venous differences reveals one of the unique features 

 of the cerebral metabolism. As shown in table 2, 

 the oxygen and glucose consumptions are 156 and 

 31 /umole per tun gm per min., respectively. Since 

 6 moles of oxygen arc required for the complete 

 oxidation of 1 mole of glucose, the rate of cerebral 

 oxygen consumption is equivalent to only 26 yumole 

 of glucose per 100 gm per min. Five /miole per 100 

 gm per min., or about 16 per cent of the total glucose 

 consumption, remain unoxidized. However, the 

 actual discrepancy from complete stoichiometric 

 equivalence is small and can, perhaps, be the result 

 of systematic errors in the analyses for oxygen and 

 glucose, particularly the latter since the analytical 

 methods employed in its determination have not been 

 specific and have included other reducing substances 

 in the blood. An alternative explanation is that of 

 Himwich & Himwich (82) who in their studies found 

 the cerebral lactate and pyruvate production to be 

 approximately equivalent to the excess glucose not 

 accounted for by the oxygen consumption. A third 

 possibility to be considered is simply that this ad- 

 ditional glucose taken up by the brain is utilized 

 not for the production of energy but for the synthesis 

 ofothei chemical constituents of the brain. 



I he combination of a cerebral respiratory quotienl 

 approximating one, an almost stoichiometric relation- 

 ship between the oxygen and glucose uptakes for the 

 Complete oxidation of the latter, and the absence of 

 ■iunilieant arteriovenous difference for am other 

 energy-rich substrate is strong evidence that the 



brain derives iis energy for normal functions almost 



exclusively from the oxidation of glucose. In this 

 respect the cerebral metabolism is quite unique in 

 thai no oilier (issue, exeepl possibly the lestes (84), 



een found to rely for energy on carbohydrate 

 alone. ( )n the oilier hand, this does not imply that the 

 pathways oi glucose metabolism in the brain lead 

 directly only to oxidation. Various chemical and 



energy transfor tions between the initial energy 



sources, oxygen and glucose, and the final products, 

 carbon dioxide and water, may occui so that various 

 intermediate compounds derived from glucose or 

 produced by the energy made available from glucose 



5.0 



4.0- 



2.0 



»• 



i 



— I— 

 10 



— r- 

 20 



— I— 

 30 



— r— 



40 



50 



60 



- 1 - 

 70 



— I 

 100 



fig. i. Changes in normal human cerebral oxygen con- 

 sumption with age. [Modified from Ketv 113, 94). Includes 

 data from following references: 37, 43, 87, 88, 100, 121, 13a, 

 157, 158, 161, 169, 175, 179; Sokoloff, Dastur, Lane & Ketv, 



unpublished observations.] 



catabolism may be the actual substances finally 

 oxidized. Indeed, Sacks (152) in studies with C u - 

 labeled glucose in human beings obtained results 

 suggesting the production of carbon dioxide by the 

 brain from sources other than glucose. The fact 

 remains, however, that so long as the oxygen and 

 glucose utilization and carbon dioxide production 

 are in such complete balance and no other energy- 

 laden substrate is taken from the blood, then the net 

 energy made available to the brain must ultimately 

 be derived from the oxidation of glucose 



The rates of cerebral metabolism presented here 

 are average values for the brain taken as a whole. 

 Although no reliable quantitative data on the meta- 

 bolic rates in vivo of the various component structures 

 of the brain are available, it is likelv that there is 

 considerable heterogeneity in (his respect. Quantita- 

 tive studies of local cerebral blood How, which in the 



normal state probably correlates well with local 

 cerebral metabolic rate, indicate considerable differ- 

 ences among the various cerebral structures. In the 



brain of the conscious cat, ketv and his associates 

 (97, io(|, 17I11 have found the blood How oi gray 

 structures to be approximately five times that of white 

 matter; and of ihe gray structures the inferior col- 

 liculus had (he highest rale, followed bv the prim.11 v 

 sensory areas oi the cortex, li is likelv that the distri- 

 bution of metal ioli< i ates is similar. 



